Different biological effects of the two protein kinase C activators bryostatin-1 and TPA on human carcinoma cell lines.

Bryostatin 1 (Bryo) is a naturally occurring macrocyclic lactone with antineoplastic activity. Like the phorbol ester 12-O-tetradecanoyl-phorbol 13-acetate (TPA) it directly activates the calcium- and phospholipid-dependent protein kinase C (PKC), thus generating a number of different cellular responses. We investigated the effects of Bryo and TPA on DNA synthesis, proliferation, viability and c-myc protooncogene expression of the human carcinoma cell lines COLO-320, MEL-HO, and KB-3-1. TPA inhibited [3H]-thymidine incorporation in all three cell lines in a dose-dependent manner, whereas Bryo only inhibited the DNA synthesis in MEL-HO, but not in KB-3-1 and COLO-320 cells. Within the concentration ranges used, TPA and Bryo were found to have a low toxicity. Counting of the cells confirmed the observed inhibition of cell proliferation. However, the enzymatic conversion of MTT, applied as a colorimetric proliferation assay, was not significantly affected by both biomodulators. Time-course experiments revealed a rapid onset of the inhibitory effect on DNA synthesis. Bryo was further able to antagonize the TPA-mediated effects on proliferation suggesting an (at least partially) different mode of action of these PKC activators. Incubation of MEL-HO and COLO-320 cells with either of the two biomodulators resulted in a rapid and strong increase of c-myc mRNA. The present study emphasizes Bryo as an interesting, natural substance for the study of PKC-mediated biological effects.